Ebook Langman’s medical embryology (12th edition): Part 1

0–2 weeks

FERTILIZATION

Not sensitive usually

EMBRYONIC DISC
DORSAL VIEW
Oropharyngeal
membrane

Epiblast

High rate of lethality
may occur

Hypoblast
Primitive streak

DORSAL ASPECT
OF EMBRYO

3–8 weeks

Oropharyngeal
membrane
Prenotochordal
cells
Primitive

node

Period of greatest
sensitivity

Each organ system will
also have a period of
peak sensitivity

Primitive
streak

Toes

FETAL MEMBRANES
IN THIRD MONTH

9–38 weeks

Decreasing sensitivity
Period of functional
maturation

Parturition

Increasing Risk

RISK OF BIRTH DEFECTS BEING INDUCED

0

3

5

8

Embryonic Period

38
Fetal Period

WEEKS GESTATION

Sadler_FM.indd i

8/25/2011 12:54:20 PM


Day 1 Fertilization

Day 2 Two-cell stage

Day 3 Morula

Day 8 Fertilization

Day 9 Trophoblast with
lacunae Enlarged blood

Day 10-11 Embryo in uterus 10-11 days after ovulation

Trophobalstic
lacunae

Day 4 Early blastocyst

Maturation of follicle Ovulation

vessels

Corpus
luteum

Corpus luteum
of pregnancy

Implanted embryo
Implantation begins
Compact layer
Cytotrophoblast

Epiblasts Hypoblast

Spongy
layer

Yolk
sac
Fibrin coagulum

Day 15 Laterality
established

Basal
layer
Exocoelomic
membrane

Day 16 Gastrulation:
Formation of germ layers

Day 17 Epiblast forms
germ layers
Primitive
node

FGF8
Neural
tube

Nodal
Lefty2
PITX2

Notochord
(SHH, T)

Lefty 1
Nodal

Snail
Node
(FGF8)

Day 22 Neural tube
closure begins

Gland

Day 18 Trilaminar
embryonic disc

Primitive
streak

Ectoderm Mesoderm

Primitive
node
Primitive
streak

Endoderm
Notochord

Invaginating
mesoderm cells

Day 23 Neural tube zippers

Day 24-25 Villus formation continues in the placenta

Anterior
neuropore

Neural fold

Syncytiotrophoblast

Pericardial
bulge

Pericardial
bulge

Villous capillary

Mesoderm core

Otic placode
Somite

Day 29 Arm and leg buds

Cytotrophoblast

Cut edge
of amnion

Cut edge
of amnion

A

Day 30 Developing face

B

Primary
villus

Posterior
neuropore

C

Secondary
villus

Day 31 Gut development

Tertiary
villus

Day 32 Embryo in
chorionic cavity
Villi

Frontonasal

prominence

Outer
cytoblast
shell

Lung bud

Nasal
placode
Foregut

Maxillary
prominence

Chorionic
plate

Mandibular
arch

Chorionic
cavity

Midgut
Cloaca

Hindgut
Decidua capsularis

Day 36 Physiological
umbilical hernia

Day 37 Developing face

Day 38 Muscle
development

m Ce
yo rv
to ica
m l
es

Occipital
myotomes
Lateral nasal
prominence

Mandibular
prominence

Day 43 Limb cartilages
and digital rays

Day 44 Developing face

Eye
muscles
IV III

II
I

T1

Urinary
bladder

Day 45 Conotruncal and
ventricular septa

Pubis

Aorta

Pulmonary
valves

Day 46

Decidua basalis
Chorion
frondosum

Decidua
parietalis

Amniotic
cavity

Chorionic
cavity

Right
artrium

Tibia

Pharyngeal
pouches

Pharyngeal
arch muscles

C1

Eye

Nasolacrimal
groove

Thoracic
myotomes

Medial nasal
prominence
Maxillary
prominence

Day 39 Endodermal
derivatives

Yolk sac
Ilium

Eye

Tricuspid
orifice

Femur

Decidua
capsularis

Uterine
cavity

Fibula
Tarsal cartilages

Nasolacrimal
groove

Chorion
laeve

Philtrum

Interventricular septum

Sadler_FM.indd ii

8/25/2011 12:54:21 PM


Day 5 Late blastocyst
Uterine
epithelium

Day 6-7 Events during first week:
Fertilization to implantation

Uterine stroma

30 hours
4

Time of
DNA
replication

6

Corpus luteum

Trophoblast
cells

Blastocyst
cavity

3 days
5

3

Development
Week 1

7 4 days
41/2-5 days
8

12-24 hours

2

Embryoblast

1 Preovulatory follicle
Fimbria
Myometrium

Outer cell mass
or trophoblast

51/2-6 days
9

Perimetrium

Day 12 Fertilization

Day 13 Uteroplacental
circulation begins

Endometrium

Day 14 Embryonic disc:
dorsal view

Primary villi

Amniotic
cavity

Cut edge
of amnion

Buccopharyngeal
membrane

Development
Week 2

Yolk sac
Chorionic
plate

Chorionic
cavity
Yolk sac

Primitive
streak

Hypoblast

Wall
of yolk
sac
Epiblast

Extraembryonic
mesoderm

Day 19 CNS induction
Cut edge
of amnion
Neural
plate

Day 20 Neurulation:
Neural folds elevate

Day 21 Transverse section
through somite region

Neural fold

Cut edge
of amnion

Somite

Intermediate
mesoderm

Development
Week 3

Neural
groove
Somite

Body
cavity

Primitive
node
Primitive
streak

Day 26 Pharyngeal arches
present
1st and 2nd
pharyngeal
arches

Anterior

neuropore

Primitive
streak

Day 27
Approx. Age
(Days)

No. of
Somites

20
21
22
23
24
25
26
27
28
30

1-4
4-7
7-10
10-13
13-17
17-20
20-23

23-26
26-29
34-35

Posterior
neuropore

Day 33 Umbilical ring
Amnion

Chorionic cavity

Day 28 Neurulation
complete
Lens
placode

Otic placode

Limb
ridge

Day 34 Optic cup and lens
placode

Development
Week 4

Day 35 Branchial arches
and clefts

Meckel's
cartilage

Yolk sac
Forebrain
Connecting
stalk

Pharyngeal
cleft

Mandibular
arch

Lens
placode

Day 40 Auricular hillocks

Development
Week 5

Optic cup

Day 41 Atrial septum
formed
Septum secundum

Hyoid arch

Day 42 Digit formation

Septum primum

Areas of cell death

Auricular
hillocks

LA

Development
Week 6

RA
3
2 4
1 5
6

RV

LV

Interventricular septum

Day 47 External genitalia
Genital
tubercle

Day 48 Facial prominences
fused

Day 49 Digits present,
eyelids forming

Genital
swelling
Urethral
fold

Development
Week 7

Lateral nasal
prominence

Medial nasal
prominence
Maxillary
prominence
Mandibular
prominence

Eye

Nasolacrimal
groove

Anal

fold

Sadler_FM.indd iii

8/25/2011 12:54:23 PM


Sadler_FM.indd iv

8/25/2011 12:54:25 PM


Sadler_FM.indd v

8/25/2011 12:54:25 PM


Acquisitions Editor: Crystal Taylor
Product Manager: Stacey Sebring
Marketing Manager: Joy Fisher-Williams
Designer: Holly Reid McLaughlin
Compositor: SPi Global
Copyright © 2012 Lippincott Williams & Wilkins, a Wolters Kluwer business.
351 West Camden Street
Baltimore, MD 21201

Two Commerce Square
2001 Market Street
Philadelphia, PA 19103

Printed in China
All rights reserved. This book is protected by copyright. No part of this book may be reproduced or transmitted
in any form or by any means, including as photocopies or scanned-in or other electronic copies, or utilized by any
information storage and retrieval system without written permission from the copyright owner, except for brief
quotations embodied in critical articles and reviews. Materials appearing in this book prepared by individuals as
part of their official duties as U.S. government employees are not covered by the above-mentioned copyright. To
request permission, please contact Lippincott Williams & Wilkins at Two Commerce Square, 2001 Market Street,
Philadelphia, PA 19103, via email at , or via website at lww.com (products and services).
Library of Congress Cataloging-in-Publication Data
Sadler, T. W. (Thomas W.)
Langman’s medical embryology. — 12th ed. / T.W. Sadler.
p. ; cm.
Medical embryology
Includes index.
ISBN 978-1-4511-1342-6
1. Embryology, Human—Textbooks. 2. Abnormalities, Human—Textbooks. I. Langman, Jan. Medical
embryology. II. Title. III. Title: Medical embryology.
[DNLM: 1. Embryology. 2. Congenital Abnormalities. QS 604]
QM601.L35 2012
612.6'4—dc23
2011025451
DISCLAIMER
Care has been taken to confirm the accuracy of the information present and to describe generally accepted
practices. However, the authors, editors, and publisher are not responsible for errors or omissions or for any
consequences from application of the information in this book and make no warranty, expressed or implied, with
respect to the currency, completeness, or accuracy of the contents of the publication. Application of this information in a particular situation remains the professional responsibility of the practitioner; the clinical treatments
described and recommended may not be considered absolute and universal recommendations.
The authors, editors, and publisher have exerted every effort to ensure that drug selection and dosage set forth
in this text are in accordance with the current recommendations and practice at the time of publication. However,
in view of ongoing research, changes in government regulations, and the constant flow of information relating to

drug therapy and drug reactions, the reader is urged to check the package insert for each drug for any change in
indications and dosage and for added warnings and precautions. This is particularly important when the recommended agent is a new or infrequently employed drug.
Some drugs and medical devices presented in this publication have Food and Drug Administration (FDA)
clearance for limited use in restricted research settings. It is the responsibility of the health care provider to ascertain the FDA status of each drug or device planned for use in their clinical practice.
To purchase additional copies of this book, call our customer service department at (800) 638-3030 or fax orders
to (301) 223-2320. International customers should call (301) 223-2300.
Visit Lippincott Williams & Wilkins on the Internet: . Lippincott Williams & Wilkins
customer service representatives are available from 8:30 am to 6:00 pm, EST.
9 8 7 6 5 4 3 2 1

Sadler_FM.indd vi

8/25/2011 12:54:28 PM


Dedication
For each and every child
and to
Dr. Tom Kwasigroch for his wonderful friendship, excellence in teaching, and dedication to
his students.

Special thanks: To Drs. David Weaver and Roger Stevenson for all of their help with the
clinical material, including providing many of the clinical figures.
To Dr. Sonja Rasmussen for her help in reviewing all of the clinical correlations and for her
expert editorial assistance.

Sadler_FM.indd vii

8/25/2011 12:54:28 PM

P

E

R

E

very student will be affected by pregnancy,
either their mother’s, since what happens in
the womb does not, necessarily, stay in the
womb, or by someone else’s. As health care professionals you will often encounter women of
childbearing age who may be pregnant, or you
may have children of your own, or maybe it is
a friend who is pregnant. In any case, pregnancy
and childbirth are relevant to all of us, and unfortunately, these processes often culminate in negative outcomes. For example, 50% of all embryos
are spontaneously aborted. Further more, prematurity and birth defects are the leading causes of
infant mortality and major contributors to disabilities. Fortunately, new strategies can improve
pregnancy outcomes, and health care professionals
have a major role to play in implementing these
initiatives. However, a basic knowledge of embryology is essential to the success of these strategies,
and with this knowledge, every health care professional can play a role in providing healthier babies.
To accomplish its goal of providing a basic
understanding of embryology and its clinical relevance, Langman’s Medical Embryology retains its
unique approach of combining an economy of
text with excellent diagrams and clinical images.
It stresses the clinical importance of the subject
by providing numerous clinical examples that
result from abnormal embryological events. The

following pedagogic features and updates in the
12th edition help facilitate student learning.
Organization of Material: Langman’s Medical
Embryology is organized into two parts. The first
provides an overview of early development from
gametogenesis through the embryonic period.
Also included in this section are chapters on placental and fetal development as well as prenatal
diagnosis and birth defects. The second part of
the text provides a description of the fundamental
processes of embryogenesis for each organ system.
Clinical Correlates: In addition to describing normal events, each chapter contains clinical
correlates that appear in highlighted boxes. This
material is designed to demonstrate the clinical
relevance of embryology and the importance
of understanding key developmental events as a
first step to improving birth outcomes and having healthier babies. Clinical pictures and case
descriptions are used to provide this information
and this material has been increased and updated
in this edition.

F

A

C

E

Genetics: Because of the increasingly important
roll of genetics and molecular biology in embryology and the study of birth defects, basic genetic and

molecular principles are discussed.The first chapter
provides an introduction to molecular pathways and
defines key terms in genetics and molecular biology.
Then, throughout the text, major signaling pathways and genes that regulate embryological development are identified and discussed.
Extensive Art Program: Nearly 400 illustrations are used to enhance understanding of the text,
including four-color line drawings, scanning electron micrographs, and clinical pictures. Additional
color pictures of clinical cases have been added to
enhance the clinical correlate sections.
Summary: At the end of each chapter is a summary that serves as a concise review of the key points
described in detail throughout the chapter. Key terms
are highlighted and defined in these summaries.
Problems to Solve: Problems related to the
key elements of each chapter are provided to
assist the student in assessing their understanding
of the material. Detailed answers are provided in
an appendix at the back of the book.
Glossary: A glossary of key terms is located
in the back of the book and has been expanded
extensively.
thePoint Web site: This site for students and
instructors provides the full text of the book and
its figures online; an interactive question bank of
USMLE board-type questions; and Simbryo animations that demonstrate normal embryological events and the origins of some birth defects.
Simbryo offers six vector art animation modules to
illustrate the complex, three-dimensional aspects
of embryology. Modules include an overview of
the normal stages of early embryogenesis, plus
development of the head and neck and the genitourinary, cardiovascular, and pulmonary systems.
Teaching aids for instructors will also be provided in the form of an image bank and a series of
lectures on the major topics in embryology presented in PowerPoint with accompanying notes.

I hope you find this edition of Langman’s
Medical Embryology to be an excellent resource for
learning embryology and its clinical significance.
Together the textbook and online site, thePoint,
are designed to provide a user-friendly and innovative approach to understanding the subject.
T.W. Sadler
Twin Bridges, MT

viii

Sadler_FM.indd viii

8/25/2011 12:54:28 PM


C

O

N

T

Preface viii
Introduction / Embryology: Clinical Relevance and
Historical Perspective xii

Part 1 General
Embryology 01
Chapter 1 / Introduction to Molecular

Regulation and Signaling 3
Gene Transcription 3
Other Regulators of Gene Expression 5
Induction and Organ Formation 5
Cell Signaling 6

Chapter 2 / Gametogenesis: Conversion
of Germ Cells into Male and Female
Gametes 10
Primordial Germ Cells 10
The Chromosome Theory of Inheritance 11
Morphological Changes During Maturation
of the Gametes 21

Chapter 3 / First Week of Development:
Ovulation to Implantation 29
Ovarian Cycle 29
Fertilization 32
Cleavage 37
Blastocyst Formation 37
Uterus at Time of Implantation 39

Chapter 4 / Second Week of
Development: Bilaminar Germ Disc 43
Day 8 43
Day 9 43
Days 11 and 12 44
Day 13 46

Chapter 5 / Third Week of

Development: Trilaminar Germ Disc 51
Gastrulation: Formation of Embryonic Mesoderm
and Endoderm 51
Formation of the Notochord 51
Establishment of the Body Axes 52
Fate Map Established During Gastrulation 57
Growth of the Embryonic Disc 57
Further Development of the Trophoblast 59

E

N

T

S

Chapter 6 / Third to Eighth Weeks: The
Embryonic Period 63
Derivatives of the Ectodermal Germ Layer 63
Derivatives of the Mesodermal Germ Layer 70
Derivatives of the Endodermal Germ Layer 78
Patterning of the Anteroposterior Axis: Regulation
by Homeobox Genes 81
External Appearance During the Second Month 81

Chapter 7 / The Gut Tube and the Body
Cavities 86
A Tube on Top of a Tube 86
Formation of the Body Cavity 87

Serous Membranes 88
Diaphragm and Thoracic Cavity 90
Formation of the Diaphragm 92

Chapter 8 / Third Month to Birth: The
Fetus and Placenta 96
Development of the Fetus 96
Fetal Membranes and Placenta 100
Chorion Frondosum and Decidua Basalis 102
Structure of the Placenta 103
Amnion and Umbilical Cord 107
Placental Changes at the End of Pregnancy 108
Amniotic Fluid 109
Fetal Membranes in Twins 110
Parturition (Birth) 115

Chapter 9 / Birth Defects and Prenatal
Diagnosis 117
Birth Defects 117
Prenatal Diagnosis 125
Fetal Therapy 128

Part 2 Systems-Based
Embryology 131
Chapter 10 / The Axial Skeleton 133
Skull 133
Vertebrae and the Vertebral Column 142
Ribs and Sternum 144

ix

Sadler_FM.indd ix

8/25/2011 12:54:29 PM


x Contents

Chapter 11 / Muscular System 145
Striated Skeletal Musculature 145
Innervation of Axial Skeletal Muscles 146
Skeletal Muscle and Tendons 148
Molecular Regulation of Muscle Development 148
Patterning of Muscles 148
Head Musculature 148
Limb Musculature 148
Cardiac Muscle 149
Smooth Muscle 149

Chapter 12 / Limbs 151
Limb Growth And Development 151
Limb Musculature 152

Chapter 13 / Cardiovascular
System 162
Establishment and Patterning of the Primary Heart
Field 162
Formation and Position of the Heart Tube 164
Formation of the Cardiac Loop 166
Molecular Regulation of Cardiac Development 169

Development of the Sinus Venosus 170
Formation of the Cardiac Septa 171
Formation of the Conducting System of the
Heart 185
Vascular Development 185
Circulation Before and After Birth 195

Chapter 14 / Respiratory System 201
Formation of the Lung Buds 201
Larynx 203
Trachea, Bronchi, And Lungs 203
Maturation of the Lungs 205

Chapter 15 / Digestive System 208
Divisions of the Gut Tube 208
Molecular Regulation of Gut Tube
Development 209
Mesenteries 210
Foregut 211
Molecular Regulation of Liver Induction 219
Pancreas 221
Midgut 222
Hindgut 229

Chapter 16 / Urogenital System 232

Pharyngeal Clefts 268
Molecular Regulation of Facial Development 268
Tongue 273
Thyroid Gland 274

Face 275
Intermaxillary Segment 278
Secondary Palate 278
Nasal Cavities 282
Teeth 283
Molecular Regulation of Tooth Development 285

Chapter 18 / Central Nervous
System 287
Spinal Cord 288
Brain 297
Molecular Regulation of Brain Development 308
Cranial Nerves 313
Autonomic Nervous System 315

Chapter 19 / Ear 321
Internal Ear 321
Middle Ear 324
External Ear 325

Chapter 20 / Eye 329
Optic Cup and Lens Vesicle 329
Retina, Iris, and Ciliary Body 331
Lens 333
Choroid, Sclera, and Cornea 333
Vitreous Body 333
Optic Nerve 334
Molecular Regulation of Eye Development 334

Chapter 21 / Integumentary System 339

Skin 339
Hair 341
Sweat Glands 342
Mammary Glands 342

Part 3 Appendix

345

Answers to Problems 347
Figure Credits 357
Glossary of Key Terms 361
Index 371

Urinary System 232
Genital System 243

Chapter 17 / Head and Neck 260
Pharyngeal Arches 262
Pharyngeal Pouches 266

Sadler_FM.indd x

8/25/2011 12:54:30 PM


Placode: A local thickening in the embryonic ectoderm layer that develops into a sensory organ or
ganglion.
ODE TO A PLACODE
Cut edge

of amnion

Neural
plate

Primitive
node

Primitive
streak

19 days

Sadler_FM.indd xi

There once was a flat sheet of cells
That were stumpy and ugly as hell;
But one day they arose, stood tall on their toes,
and declared they were the best cells of all.
Presumptuously they cried that their lineage was high
and right proudly they bragged of their codes;
But soon it was clear, they weren’t like the ear
and they were nixed in their dreams as placodes.
Semantics, they screamed, please maintain our dreams,
but their pleas were unheeded and late;
And now to this day in repast they must lay
as a misconstrued, flat neural plate!
T.W. Sadler
Twin Bridges, MT

8/25/2011 12:54:30 PM


Introduction
Embryology: Clinical
Relevance and Historical
Perspective

CLINICAL RELEVANCE
From a single cell to a baby in 9 months
(Fig. 1.1A,B); a developmental process that represents an amazing integration of increasingly complex phenomena.The study of these phenomena is
called embryology, and the field includes investigations of the molecular, cellular, and structural factors contributing to the formation of an organism.
These studies are important because they provide
knowledge essential for creating health care strategies for better reproductive outcomes. Thus, our
increasingly better understanding of embryology
has resulted in new techniques for prenatal diagnoses and treatments, therapeutic procedures to circumvent problems with infertility, and mechanisms
to prevent birth defects, the leading cause of infant
mortality. These improvements in prenatal and
reproductive health care are significant not only
for their contributions to improved birth outcomes
but also for their long-term effects postnatally. In
fact, both our cognitive capacity and our behavioral
characteristics are affected by our prenatal experiences, and factors such as maternal smoking, nutrition, stress, diabetes, etc., play a role in our postnatal
health. Furthermore, these experiences, in combination with molecular and cellular factors, determine our potential to develop certain adult diseases,
such as cancer and cardiovascular disease.Thus, our
prenatal development produces many ramifications affecting our health for both the short and
long term, making the study of embryology and
fetal development an important topic for all health
care professionals. Also, with the exception of a few
specialties, most physicians and health care workers

will have an opportunity to interact with women
of childbearing age, creating the potential for these
providers to have a major impact on the outcome
of these developmental processes and their sequelae.

A BRIEF HISTORY OF
EMBRYOLOGY
The process of progressing from a single cell
through the period of establishing organ primordia

(the first 8 weeks of human development) is
called the period of embryogenesis (sometimes called the period of organogenesis); the
period from that point on until birth is called the
fetal period, a time when differentiation continues while the fetus grows and gains weight.
Scientific approaches to study embryology have
progressed over hundreds of years. Not surprisingly, anatomical approaches dominated early
investigations. Observations were made, and
these became more sophisticated with advances
in optical equipment and dissection techniques.
Comparative and evolutionary studies were part
of this equation as scientists made comparisons
among species and so began to understand the
progression of developmental phenomena. Also
investigated were offspring with birth defects,
and these were compared to organisms with
normal developmental patterns. The study of the
embryological origins and causes for these birth
defects was called teratology.
In the 20th century, the field of experimental
embryology blossomed. Numerous experiments

were devised to trace cells during development
to determine their cell lineages.These approaches
included observations of transparent embryos
from tunicates that contained pigmented cells
that could be visualized through a microscope.
Later, vital dyes were used to stain living cells to
follow their fates. Still later in the 1960s, radioactive labels and autoradiographic techniques were
employed. One of the first genetic markers also
arose about this time with the creation of chickquail chimeras. In this approach, quail cells, which
have a unique pattern to their heterochromatin
distribution around the nucleolus, were grafted
into chick embryos at early stages of development.
Later, host embryos were examined histologically,
and the fates of the quail cells were determined.
Permutations of this approach included development of antibodies specific to quail cell antigens
that greatly assisted in the identification of these
cells. Monitoring cell fates with these and other
techniques provides valuable information about
the origins of different organs and tissues.

xii

Sadler_FM.indd xii

8/25/2011 12:54:31 PM


Introduction

Grafting experiments also provided the first

insights into signaling between tissues. Examples
of such experiments included grafting the primitive node from its normal position on the body
axis to another and showing that this structure
could induce a second body axis. In another
example, employing developing limb buds, it was
shown that if a piece of tissue from the posterior axial border of one limb was grafted to the
anterior border of a second limb, then digits on
the host limb would be duplicated as the mirror image of each other. This posterior signaling region was called the zone of polarizing
activity (ZPA), and it is now known that the
signaling molecule is sonic hedgehog (SHH).
About this same time (1961), the science of
teratology became prominent because of the drug
thalidomide that was given as an antinauseant
and sedative to pregnant women. Unfortunately,
the drug caused birth defects, including unique
abnormalities of the limbs in which one or more
limbs was absent (amelia) or was lacking the
long bones such that only a hand or foot was
attached to the torso (phocomelia). The association between the drug and birth defects was

Sadler_FM.indd xiii

Embryology: Clinical Relevance and Historical Perspective xiii

recognized independently by two clinicians,
W. Lenz and W. McBride and showed that the
conceptus was vulnerable to maternal factors
that crossed the placenta. Soon, numerous animal
models demonstrating an association between
environmental factors, drugs, and genes provided

further insights between developmental events
and the origin of birth defects.
Today, molecular approaches have been added
to the list of experimental paradigms used to study
normal and abnormal development. Numerous
means of identifying cells using reporter genes,
fluorescent probes, and other marking techniques
have improved our ability to map cell fates. Using
other techniques to alter gene expression, such
as knockout, knock-in, and antisense technologies has created new ways to produce abnormal
development and allowed the study of a single
gene’s function in specific tissues. Thus, the
advent of molecular biology has advanced the
field of embryology to the next level, and as we
decipher the roles of individual genes and their
interplay with environmental factors, our understanding of normal and abnormal developmental
processes progresses.

8/25/2011 12:54:31 PM


Sadler_FM.indd xiv

8/25/2011 12:54:31 PM


Sadler_Chap01.indd 1

8/25/2011 3:23:38 PM

Sadler_Chap01.indd 2

8/25/2011 3:23:44 PM


Chapter 1
Introduction to Molecular
Regulation and Signaling

M

olecular biology has opened the doors to
new ways to study embryology and to
enhance our understanding of normal and
abnormal development. Sequencing the human
genome, together with creating techniques to
investigate gene regulation at many levels of complexity, has taken embryology to the next level.
Thus, from the anatomical to the biochemical to
the molecular level, the story of embryology has
progressed, and each chapter has enhanced our
knowledge.
There are approximately 23,000 genes in
the human genome, which represents only one
fifth of the number predicted prior to completion of the Human Genome Project. Because
of various levels of regulation, however, the
number of proteins derived from these genes is
closer to the original predicted number of genes.
What has been disproved is the one-gene–oneprotein hypothesis. Thus, through a variety of
mechanisms, a single gene may give rise to many

proteins.
Gene expression can be regulated at several
levels: (1) different genes may be transcribed, (2)
nuclear deoxyribonucleic acid (DNA) transcribed
from a gene may be selectively processed to regulate which RNAs reach the cytoplasm to become
messenger RNAs (mRNAs), (3) mRNAs may be
selectively translated, and (4) proteins made from
the mRNAs may be differentially modified.

GENE TRANSCRIPTION
Genes are contained in a complex of DNA and
proteins (mostly histones) called chromatin, and
its basic unit of structure is the nucleosome
(Fig. 1.1). Each nucleosome is composed of an
octamer of histone proteins and approximately
140 base pairs of DNA. Nucleosomes themselves
are joined into clusters by binding of DNA existing between nucleosomes (linker DNA) with
other histone proteins (H1 histones; Fig. 1.1).
Nucleosomes keep the DNA tightly coiled, such
that it cannot be transcribed. In this inactive state,
chromatin appears as beads of nucleosomes on a

string of DNA and is referred to as heterochromatin. For transcription to occur, this DNA
must be uncoiled from the beads. In this uncoiled
state, chromatin is referred to as euchromatin.
Genes reside within the DNA strand and
contain regions called exons, which can be
translated into proteins, and introns, which are
interspersed between exons and which are not
transcribed into proteins (Fig. 1.2). In addition

to exons and introns, a typical gene includes the
following: a promoter region that binds RNA
polymerase for the initiation of transcription; a transcription initiation site; a translation initiation site to designate the first amino
acid in the protein; a translation termination
codon; and a 3′ untranslated region that includes
a sequence (the poly A addition site) that assists
with stabilizing the mRNA, allows it to exit the
nucleus, and permits it to be translated into protein (Fig. 1.2). By convention, the 5′ and the 3′
regions of a gene are specified in relation to the
RNA transcribed from the gene. Thus, DNA is
transcribed from the 5′ to the 3′ end, and the
promoter region is upstream from the transcription initiation site (Fig. 1.2). The promoter
region, where the RNA polymerase binds, usually contains the sequence TATA, and this site
Histone complex
DNA

Nucleosome
H1
histones

Linker
DNA

Figure 1.1 Drawing showing nucleosomes that form the
basic unit of chromatin. Each nucleosome consists of an
octamer of histone proteins and approximately 140 base
pairs of DNA. Nucleosomes are joined into clusters by
linker DNA and other histone proteins.

3

Sadler_Chap01.indd 3

8/25/2011 3:23:44 PM


4 Part 1 General Embryology
Promoter
region

Exon 1

Intron 1

Exon 2 Intron 2 Exon 3 Intron 3

Exon 4
3' untranslated region

TATA
box

Enhancer
sequence

Translation
initiation
codon

Translation

Transcription
termination
termination
site
Poly A
addition site

Figure 1.2 Drawing of a “typical” gene showing the promoter region containing the TATA box; exons that contain DNA
sequences that are translated into proteins; introns; the transcription initiation site; the translation initiation site that
designates the code for the first amino acid in a protein; and the 3′ untranslated region that includes the poly A addition site
that participates in stabilizing the mRNA, allows it to exit the nucleus, and permits its translation into a protein.

is called the TATA box (Fig. 1.2). In order to
bind to this site, however, the polymerase requires
additional proteins called transcription factors
(Fig. 1.3).Transcription factors also have a specific
DNA-binding domain plus a transactivating domain that activates or inhibits transcription of the gene whose promoter or enhancer it
has bound. In combination with other proteins,
transcription factors activate gene expression
by causing the DNA nucleosome complex to
unwind, by releasing the polymerase so that it
can transcribe the DNA template, and by preventing new nucleosomes from forming.
Enhancers are regulatory elements of DNA
that activate utilization of promoters to control
their efficiency and the rate of transcription from
the promoter. Enhancers can reside anywhere
along the DNA strand and do not have to reside
close to a promoter. Like promoters, enhancers
bind transcription factors (through the transcription factor’s transactivating domain) and are used
to regulate the timing of a gene’s expression and

its cell-specific location. For example, separate
enhancers in a gene can be used to direct the
same gene to be expressed in different tissues.
The PAX6 transcription factor, which participates in pancreas, eye, and neural tube development, contains three separate enhancers, each
of which regulates the gene’s expression in the

RNA Polymerase II

appropriate tissue. Enhancers act by altering
chromatin to expose the promoter or by facilitating binding of the RNA polymerase. Sometimes,
enhancers can inhibit transcription and are called
silencers. This phenomenon allows a transcription factor to activate one gene while silencing
another by binding to different enhancers. Thus,
transcription factors themselves have a DNAbinding domain specific to a region of DNA plus
a transactivating domain that binds to a promoter
or an enhancer and activates or inhibits the gene
regulated by these elements.

DNA Methylation Represses
Transcription
Methylation of cytosine bases in the promoter
regions of genes represses transcription of those
genes. Thus, some genes are silenced by this
mechanism. For example, one of the X chromosomes in each cell of a female is inactivated
(X chromosome inactivation) by this methylation mechanism. Similarly, genes in different
types of cells are repressed by methylation, such
that muscle cells make muscle proteins (their
promoter DNA is mostly unmethylated), but not
blood proteins (their DNA is highly methylated).
In this manner, each cell can maintain its characteristic differentiated state. DNA methylation

is also responsible for genomic imprinting in

RNA Polymerase II

DNA

TATA

Transcription
factor protein
complex

Transcription
initiation site

RNA transcript

Figure 1.3 Drawing showing binding of RNA polymerase II to the TATA box site of the promoter region of a gene. This
binding requires a complex of proteins plus an additional protein called a transcription factor. Transcription factors have their
own specific DNA-binding domain and function to regulate gene expression.

Sadler_Chap01.indd 4

8/25/2011 3:23:53 PM


Chapter 1

which only a gene inherited from the father or
the mother is expressed, while the other gene

is silenced. Approximately 40 to 60 human
genes are imprinted and their methylation patterns are established during spermatogenesis and
oogenesis. Methylation silences DNA by inhibiting binding of transcription factors or by altering histone binding resulting in stabilization of
nucleosomes and tightly coiled DNA that cannot
be transcribed.

OTHER REGULATORS OF GENE
EXPRESSION
The initial transcript of a gene is called nuclear
RNA (nRNA) or sometimes premessenger RNA.
nRNA is longer than mRNA because it contains introns that are removed (spliced out) as
the nRNA moves from the nucleus to the cytoplasm. In fact, this splicing process provides a
means for cells to produce different proteins from
a single gene. For example, by removing different
introns, exons are “spliced” in different patterns,
a process called alternative splicing (Fig. 1.4).
The process is carried out by spliceosomes,
which are complexes of small nuclear RNAs
(snRNAs) and proteins that recognize specific
splice sites at the 5′ or the 3′ ends of the nRNA.
Proteins derived from the same gene are called
splicing isoforms (also called splice variants or alternative splice forms), and these
afford the opportunity for different cells to use
the same gene to make proteins specific for that
cell type. For example, isoforms of the WT1 gene
have different functions in gonadal versus kidney
development.
Even after a protein is made (translated), there
may be post-translational modifications that
affect its function. For example, some proteins

5' untranslated
region

Exons

Introduction to Molecular Regulation and Signaling

5

have to be cleaved to become active, or they
might have to be phosphorylated. Others need
to combine with other proteins or be released
from sequestered sites or be targeted to specific
cell regions. Thus, there are many regulatory levels for synthesizing and activating proteins, such
that although only 23,000 genes exist, the potential number of proteins that can be synthesized is
probably closer to five times the number of genes.

INDUCTION AND ORGAN
FORMATION
Organs are formed by interactions between cells
and tissues. Most often, one group of cells or tissues
causes another set of cells or tissues to change
their fate, a process called induction. In each such
interaction, one cell type or tissue is the inducer
that produces a signal, and one is the responder
to that signal. The capacity to respond to such
a signal is called competence, and competence
requires activation of the responding tissue by a
competence factor. Many inductive interactions occur between epithelial and mesenchymal
cells and are called epithelial–mesenchymal

interactions (Fig. 1.5). Epithelial cells are joined
together in tubes or sheets, whereas mesenchymal
cells are fibroblastic in appearance and dispersed
in extracellular matrices (Fig. 1.5). Examples of
epithelial–mesenchymal interactions include the
following: gut endoderm and surrounding mesenchyme to produce gut-derived organs, including the liver and pancreas; limb mesenchyme
with overlying ectoderm (epithelium) to produce limb outgrowth and differentiation; and
endoderm of the ureteric bud and mesenchyme
from the metanephric blastema to produce
nephrons in the kidney. Inductive interactions
can also occur between two epithelial tissues,
Tissue specific
Exon (bone)

Introns

3' untranslated
region

Hypothetical
gene
Protein I
Protein II
(bone)
Protein III

Figure 1.4 Drawing of a hypothetical gene illustrating the process of alternative splicing to form different proteins from
the same gene. Spliceosomes recognize specific sites on the initial transcript of nRNA from a gene. Based on these sites,
different introns are “spliced out” to create more than one protein from a single gene. Proteins derived from the same gene
are called splicing isoforms.

Sadler_Chap01.indd 5

8/25/2011 3:23:53 PM


6 Part 1 General Embryology

to interact with other cells, or by juxtacrine
interactions, which do not involve diffusable
proteins. The diffusable proteins responsible for
paracrine signaling are called paracrine factors or growth and differentiation factors
(GDFs).

Mesenchyme

Epithelium

Signal Transduction Pathways
Paracrine Signaling
Paracrine factors act by signal transduction
pathways either by activating a pathway directly
or by blocking the activity of an inhibitor of a
pathway (inhibiting an inhibitor, as is the case
with hedgehog signaling). Signal transduction
pathways include a signaling molecule (the
ligand) and a receptor (Fig. 1.6). The receptor
spans the cell membrane and has an extracellular domain (the ligand-binding region), a
transmembrane domain, and a cytoplasmic
domain. When a ligand binds its receptor, it

induces a conformational change in the receptor that activates its cytoplasmic domain. Usually,
the result of this activation is to confer enzymatic activity to the receptor, and most often
this activity is a kinase that can phosphorylate
other proteins using ATP as a substrate. In turn,
phosphorylation activates these proteins to phosphorylate additional proteins, and thus a cascade
of protein interactions is established that ultimately activates a transcription factor. This
transcription factor then activates or inhibits
gene expression. The pathways are numerous and

Figure 1.5 Drawing illustrating an epithelial–mesenchymal
interaction. Following an initial signal from one tissue, a
second tissue is induced to differentiate into a specific
structure. The first tissue constitutes the inducer, and the
second is the responder. Once the induction process is
initiated, signals (arrows) are transmitted in both directions
to complete the differentiation process.

such as induction of the lens by epithelium of
the optic cup. Although an initial signal by the
inducer to the responder initiates the inductive
event, crosstalk between the two tissues or cell
types is essential for differentiation to continue
(Fig. 1.5, arrows).

CELL SIGNALING
Cell-to-cell signaling is essential for induction,
for conference of competency to respond, and for
crosstalk between inducing and responding cells.
These lines of communication are established by
paracrine interactions, whereby proteins synthesized by one cell diffuse over short distances

Ligand
Receptor complex
Cell membrane
P
P

Nuclear
pores

P
P

P

Activated
(kinase) region

Activated protein

Cytoplasm
P

Activated protein
complex
Activated protein
complex acts as a
transcription factor

P

Nucleus

Figure 1.6 Drawing of a typical signal transduction pathway involving a ligand and its receptor. Activation of the receptor
is conferred by binding to the ligand. Typically, the activation is enzymatic involving a tyrosine kinase, although other enzymes
may be employed. Ultimately, kinase activity results in a phosphorylation cascade of several proteins that activates a
transcription factor for regulating gene expression.

Sadler_Chap01.indd 6

8/25/2011 3:23:53 PM


Chapter 1

complex and in some cases are characterized by
one protein inhibiting another that in turn activates another protein (much like the situation
with hedgehog signaling).
Juxtacrine Signaling
Juxtacrine signaling is mediated through signal transduction pathways as well but does not
involve diffusable factors. Instead, there are three
ways juxtacrine signaling occurs: (1) A protein
on one cell surface interacts with a receptor on
an adjacent cell in a process analogous to paracrine signaling (Fig. 1.6). The Notch pathway
represents an example of this type of signaling. The Notch receptor protein extends across
the cell membrane and binds to cells that have
Delta, Serrate, or Jagged proteins in their
cell membranes. Binding of one of these proteins to Notch causes a conformational change
in the Notch protein such that part of it on the
cytoplasmic side of the membrane is cleaved.The

cleaved portion then binds to a transcription factor to activate gene expression. Notch signaling
is especially important in neuronal differentiation, blood vessel specification, and somite segmentation. (2) Ligands in the extracellular matrix
secreted by one cell interact with their receptors
on neighboring cells. The extracellular matrix
is the milieu in which cells reside. This milieu
consists of large molecules secreted by cells
including collagen, proteoglycans (chondroitin sulfates, hyaluronic acid, etc.), and glycoproteins, such as fibronectin and laminin.
These molecules provide a substrate for cells on
which they can anchor or migrate. For example,
laminin and type IV collagen are components
of the basal lamina for epithelial cell attachment, and fibronectin molecules form scaffolds
for cell migration. Receptors that link extracellular molecules such as fibronectin and laminin to
cells are called integrins. These receptors “integrate” matrix molecules with a cell’s cytoskeletal machinery (e.g., actin microfilaments)
thereby creating the ability to migrate along
matrix scaffolding by using contractile proteins,
such as actin. Also, integrins can induce gene
expression and regulate differentiation as in the
case of chondrocytes that must be linked to the
matrix to form cartilage. (3) There is direct transmission of signals from one cell to another by
gap junctions. These junctions occur as channels between cells through which small molecules and ions can pass. Such communication is
important in tightly connected cells like epithelia
of the gut and neural tube because they allow
these cells to act in concert. The junctions themselves are made of connexin proteins that form

Sadler_Chap01.indd 7

Introduction to Molecular Regulation and Signaling

7

a channel, and these channels are “connected”
between adjacent cells.
It is important to note that there is a great
amount of redundancy built into the process of
signal transduction. For example, paracrine signaling molecules often have many family members such that other genes in the family may
compensate for the loss of one of their counterparts. Thus, the loss of function of a signaling
protein through a gene mutation does not necessarily result in abnormal development or death.
In addition, there is crosstalk between pathways,
such that they are intimately interconnected.
These connections provide numerous additional
sites to regulate signaling.

Paracrine Signaling Factors
There are a large number of paracrine signaling
factors acting as ligands, which are also called
GDFs. Most are grouped into four families, and
members of these same families are used repeatedly to regulate development and differentiation
of organ systems. Furthermore, the same GDFs
regulate organ development throughout the animal kingdom from Drosophila to humans. The
four groups of GDFs include the fibroblast
growth factor (FGF), WNT, hedgehog, and
transforming growth factor-b (TGF-b)
families. Each family of GDFs interacts with its
own family of receptors, and these receptors are
as important as the signal molecules themselves
in determining the outcome of a signal.
Fibroblast Growth Factors
Originally named because they stimulate the
growth of fibroblasts in culture, there are now
approximately two dozen FGF genes that have

been identified, and they can produce hundreds
of protein isoforms by altering their RNA splicing or their initiation codons. FGF proteins
produced by these genes activate a collection
of tyrosine receptor kinases called fibroblast growth factor receptors (FGFRs). In
turn, these receptors activate various signaling
pathways. FGFs are particularly important for
angiogenesis, axon growth, and mesoderm differentiation. Although there is redundancy in
the family, such that FGFs can sometimes substitute for one another, individual FGFs may be
responsible for specific developmental events. For
example, FGF8 is important for development of
the limbs and parts of the brain.
Hedgehog Proteins
The hedgehog gene was named because it coded
for a pattern of bristles on the leg of Drosophila
that resembled the shape of a hedgehog. In

8/25/2011 3:23:54 PM


8 Part 1 General Embryology

mammals, there are three hedgehog genes,
Desert, Indian, and sonic hedgehog. Sonic hedgehog
is involved in a number of developmental events
including limb patterning, neural tube induction and patterning, somite differentiation, gut
regionalization, and others. The receptor for the
hedgehog family is Patched, which binds to a
protein called Smoothened. The Smoothened
protein transduces the hedgehog signal, but it
is inhibited by Patched until the hedgehog protein binds to this receptor. Thus, the role of the

paracrine factor hedgehog in this example is to
bind to its receptor to remove the inhibition of a
transducer that would normally be active, not to
activate the transducer directly.
WNT Proteins
There are at least 15 different WNT genes that
are related to the segment polarity gene, wingless
in Drosophilia.Their receptors are members of the
frizzled family of proteins. WNT proteins are
involved in regulating limb patterning, midbrain
development, and some aspects of somite and
urogenital differentiation among other actions.

The TGF-b Superfamily
The TGF-b superfamily has more than 30 members and includes the TGF-bs, the bone morphogenetic proteins, the activin family, the
Müllerian inhibiting factor (MIF, anti-Müllerian hormone), and others. The first member
of the family, TGF-b1, was isolated from virally
transformed cells.TGF-b members are important
for extracellular matrix formation and epithelial
branching that occurs in lung, kidney, and salivary
gland development. The BMP family induces
bone formation and is involved in regulating cell
division, cell death (apoptosis), and cell migration
among other functions.

Other Paracrine Signaling Molecules
Another group of paracrine signaling molecules
important during development are neurotransmitters, including serotonin and norepinephrine,
that act as ligands and bind to receptors just as proteins do.These molecules are not just transmitters
for neurons, but also provide important signals for

embryological development. For example, serotonin (5HT) acts as a ligand for a large number of
receptors, most of which are G protein–coupled
receptors. Acting through these receptors, 5HT
regulates a variety of cellular functions, including
cell proliferation and migration, and is important for establishing laterality, gastrulation, heart
development, and other processes during early
stages of differentiation. Norepinephrine also acts
through receptors and appears to play a role in

Sadler_Chap01.indd 8

apoptosis (programmed cell death) in the
interdigital spaces and in other cell types.

Summary
During the past century, embryology has progressed from an observational science to one
involving sophisticated technological and molecular advances. Together, observations and modern techniques provide a clearer understanding
of the origins of normal and abnormal development and, in turn, suggest ways to prevent and
treat birth defects. In this regard, knowledge of
gene function has created entire new approaches
to the subject.
There are approximately 23,000 genes in
the human genome, but these genes code for
approximately 100,000 proteins. Genes are
contained in a complex of DNA and proteins
called chromatin, and its basic unit of structure
is the nucleosome. Chromatin appears tightly
coiled as beads of nucleosomes on a string and
is called heterochromatin. For transcription to
occur, DNA must be uncoiled from the beads

as euchromatin. Genes reside within strands
of DNA and contain regions that can be translated into proteins, called exons, and untranslatable regions, called introns. A typical gene also
contains a promoter region that binds RNA
polymerase for the initiation of transcription; a
transcription initiation site, to designate the
first amino acid in the protein; a translation termination codon; and a 3′ untranslated region
that includes a sequence (the poly A addition
site) that assists with stabilization of the mRNA.
The RNA polymerase binds to the promoter
region that usually contains the sequence TATA,
the TATA box. Binding requires additional proteins called transcription factors. Methylation
of cytosine bases in the promoter region silences
genes and prevents transcription. This process is
responsible for X chromosome inactivation
whereby the expression of genes on one of the X
chromosomes in females is silenced and also for
genomic imprinting in which either a paternal
or a maternal gene’s expression is repressed.
Different proteins can be produced from a
single gene by the process of alternative splicing
that removes different introns using spliceosomes. Proteins derived in this manner are called
splicing isoforms or splice variants. Also,
proteins may be altered by post-translational
modifications, such as phosphorylation or
cleavage.
Induction is the process whereby one group
of cells or tissues (the inducer) causes another

8/25/2011 3:23:54 PM

Chapter 1

group (the responder) to change their fate. The
capacity to respond is called competence and
must be conferred by a competence factor.
Many inductive phenomena involve epithelial–
mesenchymal interactions.
Signal transduction pathways include a
signaling molecule (the ligand) and a receptor. The receptor usually spans the cell membrane and is activated by binding with its specific
ligand. Activation usually involves the capacity
to phosphorylate other proteins, most often as
a kinase. This activation establishes a cascade of
enzyme activity among proteins that ultimately
activates a transcription factor for initiation of
gene expression.
Cell-to-cell signaling may be paracrine,
involving diffusable factors, or juxtacrine,
involving a variety of nondiffusable factors.
Proteins responsible for paracrine signaling are
called paracrine factors or growth and differentiation factors (GDFs). There are four
major families of GDFs: FGFs, WNTs, hedgehogs, and TGF-bs. In addition to proteins,

Sadler_Chap01.indd 9

Introduction to Molecular Regulation and Signaling

9

neurotransmitters, such as serotonin (5HT)

and norepinephrine, also act through paracrine signaling, serving as ligands and binding to
receptors to produce specific cellular responses.
Juxtacrine factors may include products of the
extracellular matrix, ligands bound to a cell’s
surface, and direct cell-to-cell communications.

Problems to Solve
1. What is meant by “competence to respond”
as part of the process of induction? What
tissues are most often involved in induction?
Give two examples.
2. Under normal conditions, FGFs and their
receptors (FGFRs) are responsible for growth
of the skull and development of the cranial
sutures. How might these signaling pathways
be disrupted? Do these pathways involve
paracrine or juxtacrine signaling? Can you
think of a way that loss of expression of one
FGF might be circumvented?

8/25/2011 3:23:54 PM